A plasma jet plug is known as an example of a spark plug for igniting an air-fuel mixture in an internal combustion engine (for example, PTL 1). The plasma jet plug has a discharge space (also referred to as a cavity) which is surrounded by an insulator, such as a ceramic, and in which a gap is provided between a center electrode and a ground electrode. When a spark is generated (discharge occurs) in the gap, the gas in the cavity is excited and plasma is generated in the cavity. The generated plasma is ejected out of the cavity, so that the air-fuel mixture is ignited. The ejected plasma is capable of quickly reaching positions far from the plasma jet plug. Therefore, unlike a spark plug that directly ignites the air-fuel mixture with a spark (discharge), the plasma jet plug is capable of appropriately igniting a lean air-fuel mixture having a high air/fuel ratio.